Solvent Induced Helix Folding of Defined Indolenine Squaraine Oligomers

A protecting group strategy was employed to synthesise a series of indolenine squaraine dye oligomers up to the nonamer. The longer oligomers show a distinct solvent dependence of the absorption spectra, that is, either a strong blue shift or a strong red shift of the lowest energy bands in the near infrared spectral region. This behaviour is explained by exciton coupling theory as being due to H- or J-type coupling of transition moments. The H-type coupling is a consequence of a helix folding in solvents with a small Hansen dispersity index. DOSY NMR, small angle neutron scattering (SANS), quantum chemical and force field calculations agree upon a helix structure with an unusually large pitch and open voids that are filled with solvent molecules, thereby forming a kind of clathrate. The thermodynamic parameters of the folding process were determined by temperature dependent optical absorption spectra.     

α‐Chymotrypsin‐Catalyzed Reaction Confined in Block‐Copolymer Vesicles

Herein the reactivity of the enzyme α‐chymotrypsin in the confinement of polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) vesicles was investigated. Enzyme and substrate molecules were encapsulated in PS‐b‐PAA vesicles with internal diameters ranging from 26 nm to 165 nm during the formation of the vesicles. While the loading efficiencies of enzyme and substrate molecules were practically identical for vesicles of identical size, they were found to increase with decreasing vesicle size. The kinetics of the α‐chymotrypsin catalyzed hydrolysis of N‐succinyl‐Ala‐Ala‐Phe‐7‐amido‐4‐methylcoumarin (AMC) was evaluated following the increase of the absorption of the product 7‐amino‐4‐methylcoumarin by UV/Vis spectroscopy. The values of the catalytic turnover number obtained for reactions inside vesicles with different sizes showed an increase of up to fourteen times compared to the bulk value with decreasing vesicle volume, while the values of the Michaelis–Menten constant decreased, respectively. This increase in reactivity of α‐chymotrypsin is attributed to the effect of vesicle–wall interactions in the finite encapsulated space, where the reagents could diffuse, leading to enhanced collision frequencies.     

Effects of Coke Deposits on the Catalytic Performance of Large Zeolite H‐ZSM‐5 Crystals during Alcohol‐to‐Hydrocarbon Reactions as Investigated by a Combination of Optical Spectroscopy and Microscopy

The catalytic activity of large zeolite H‐ZSM‐5 crystals in methanol (MTO) and ethanol‐to‐olefins (ETO) conversions was investigated and, using operando UV/Vis measurements, the catalytic activity and deactivation was correlated with the formation of coke. These findings were related to in situ single crystal UV/Vis and confocal fluorescence micro‐spectroscopy, allowing the observation of the spatiotemporal formation of intermediates and coke species during the MTO and ETO conversions. It was observed that rapid deactivation at elevated temperatures was due to the fast formation of aromatics at the periphery of the H‐ZSM‐5 crystals, which are transformed into more poly‐aromatic coke species at the external surface, preventing the diffusion of reactants and products into and out of the H‐ZSM‐5 crystal. Furthermore, we were able to correlate the operando UV/Vis spectroscopy results observed during catalytic testing with the single crystal in situ results.     

Small‐Peptide‐Based Organogel Kit: Towards the Development of Multicomponent Self‐Sorting Organogels

The results presented here highlight the extremely useful nature of ultra‐short peptides as building blocks in the development of smart multicomponent supramolecular devices. A facile bottom‐up strategy for the synthesis of a small library of stimuli‐responsive smart organogelators has been proposed based on the predictive self‐assembly of ultra‐short peptides. More importantly, the narcissistic self‐sorting of the gelators has been evaluated as a simple method for the efficient co‐assembly of a donor–acceptor dual‐component gel, allowing the investigation of possible future applications of similar systems in the development of a supramolecular photo‐conversion device. Interestingly, it was observed that the self‐organization of the components can lead to highly ordered systems in which discrimination between compatible and non‐compatible building blocks directs the effective organization of the chromophores and gives rise to the formation of an excited‐state complex with exciplex‐like emission. The current report may prove important in the development of organogel‐based multicomponent smart devices.     

Chiral interactions in azobenzene dimers: a combined experimental and theoretical study

To investigate interchromophore interactions in azobenzene polymers, we have undertaken a thorough spectroscopic analysis of the azodye [(S)-3-pivaloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine] by modeling the repeating unit of poly[(S)-3-methacryloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine) and its dimeric derivative whose synthesis is presented here. The analysis of the electronic and Raman spectra of the azodye in several solvents is based on a previously proposed model for polar chromophores in solution. Electronic and CD spectra of the dimeric unit are collected and analyzed within the framework of a new model. On the basis of the information collected from the spectroscopic analysis of the solvated dye, this model accounts for interchromophore interactions in the dimer. The large CD signal measured for the dimer (amounting to about a third of the signal measured for the polymer) suggests the presence of important chiral interactions in the dimeric unit, and is modeled in terms of a right-handed relative orientation of the two chromophores.     

Dimeric Calixarenes: A New Family of Major‐Groove Binders

A new class of potent DNA binding agents is presented. Dimeric calix[4]arenes with cationic groups at their upper rims and flexible alkyl bridges can be synthesized from triply acyl‐protected calix[4]arene tetramines in relatively short synthetic sequences (3–5 steps). The compounds attach themselves to double‐stranded nucleic acids in a noncovalent fashion, with micro‐ to nanomolar affinities. Guanidinium headgroups with their extended hydrogen‐bonding “fingers” are more powerful than ammonium groups, and the benzylamine series is superior to the anilinium series (see below). The new ligands easily distinguish between RNA and various DNA types, and produce characteristic changes in UV/Vis, fluorescence, CD, as well as NMR spectra. Especially extended oligonucleotides of more than 100 base pairs are bound with affinities increasing from RNA (10 μM K_d)<AT‐rich (1 μM )<GC‐rich DNA double strands (100–10 nM ). Ethidium bromide displacement studies confirm this order. CE₅₀ values are remarkably low (1–4 μM ), and are more than 300 times lower than that of spermine, which is a typical backbone binder. Stoichiometries are rather high (one calixarene dimer per two BP), suggesting a potential aggregation of bound ligands inside the major groove. Most UV/Vis melting curves display an inverted shape, and start from drastically enhanced absorption intensities for the DNA complexes. DAPI displacement studies prove that up to one equivalent of calixarene dimer can be accommodated in the dye‐loaded DNA. RNA complexation by calixarene dimers is accompanied by a drastic CD spectral transition from the typical A‐form to a perfect B‐signature, providing further experimental evidence for major‐groove binding. The orientation of the ligands can be deduced from NMR titrations and is reproduced in Monte‐Carlo simulations on 1:1 complexes in water.     

A Self‐Assembling Polythiophene Functionalised with a Cysteine Moiety

A new copolymer bearing a cysteine moiety, designed for molecular interaction, metal‐ion detection, and chiral recognition, was synthesised starting from the dibromo derivative of methyl N‐(tert‐butoxycarbonyl)‐S‐thien‐3‐ylcysteinate and distannylthiophene through a Stille coupling reaction. UV‐vis spectroscopy, circular dichroism, NMR spectroscopy, and gel permeation chromatography analyses evidenced that this polymer is able to form self‐assembling structures, through the formation of a hydrogen‐bond network, not only in the solid state but also in solution.

     

Affinity,Speciation, and Molecular Features of Copper(II) Complexes with a Prion Tetraoctarepeat Domain in Aqueous Solution: Insights into Old and New Results

Characterization of the copper(II) complexes formed with the tetraoctarepeat peptide at low and high metal‐to‐ligand ratios and in a large pH range, would provide a breakthrough in the interpretation of biological relevance of the different metal complexes of copper(II)‐tetraoctarepeat system. In the present work, the potentiometric, UV/Vis, circular dichroism (CD), and electron paramagnetic resonance (EPR) studies were carried out on copper(II) complexes with a PEG‐ylated derivative of the tetraoctarepeats peptide sequence (Ac‐PEG₂₇‐(PHGGGWGQ)₄‐NH₂) and the peptide Ac‐(PHGGGWGQ)₂‐NH₂. Conjugation of tetraoctarepeat peptide sequence with polyethyleneglycol improved the solubility of the copper(II) complexes. The results enable a straightforward explanation of the conflicting results originated from the underestimation of all metal–ligand equilibria and the ensuing speciation. A complete and reliable speciation is therefore obtained with the released affinity and binding details of the main complexes species formed in aqueous solution. The results contribute to clarify the discrepancies of several studies in which the authors ascribe the redox activity of copper(II)‐tetraoctarepeat system considering only the average effects of several coexisting species with very different stoichiometries and binding modes.     

Helix Nucleation by the Smallest Known α‐Helix in Water

Cyclic pentapeptides (e.g. Ac‐(cyclo‐1,5)‐[KAXAD]‐NH₂; X=Ala, 1 ; Arg, 2 ) in water adopt one α‐helical turn defined by three hydrogen bonds. NMR structure analysis reveals a slight distortion from α‐helicity at the C‐terminal aspartate caused by torsional restraints imposed by the K(i)–D(i+4) lactam bridge. To investigate this effect on helix nucleation, the more water‐soluble 2 was appended to N‐, C‐, or both termini of a palindromic peptide ARAARAARA (≤5 % helicity), resulting in 67, 92, or 100 % relative α‐helicity, as calculated from CD spectra. From the C‐terminus of peptides, 2 can nucleate at least six α‐helical turns. From the N‐terminus, imperfect alignment of the Asp5 backbone amide in 2 reduces helix nucleation, but is corrected by a second unit of 2 separated by 0–9 residues from the first. These cyclic peptides are extremely versatile helix nucleators that can be placed anywhere in 5–25 residue peptides, which correspond to most helix lengths in protein–protein interactions.     

Photoinduced Formation of an Azobenzene‐Based CD‐Active Supramolecular Cyclic Dimer

A series of new photo‐responsive amino acid‐derived azobenzenedicarboxylic acid derivatives (S)‐ 1 a – e were synthesized. Compound (S)‐ 1 a in the trans form exhibited no circular dichroism (CD) signal in DMF under ambient conditions, whereas intense Cotton effects were observed upon UV irradiation, indicating the formation of a chiral supramolecular structure in the cis form. The CD signals disappeared when trifluoroacetic acid (TFA) was added to the solution. The ester counterpart [(S)‐ 1 a′ ] showed no CD signal. Hydrogen bonding between the carboxy groups seemed necessary for constructing the supramolecular structure. The kinetic studies of cis to trans isomerization of (S)‐ 1 a demonstrated that the formation of a chiral supramolecule enhances the stability of the cis‐azobenzene structure. The ESI mass spectrum of stilbenedicarboxylic acid (S)‐ 4 , an analogue of (S)‐ 1 b , confirmed the formation of a dimer. A theoretical CD study revealed that (S)‐ 1 a in the cis form should be present as a cyclic chiral dimer.     

Anion Coordination of Bis‐bisurea Ligands: Aggregation of Dihydrogen Phosphate Anion into Oligomers and Infinite Chains

A series of alkanediyl‐spaced bis‐bisurea ligands ( L² – L⁴ ) were synthesized and their anion coordination behavior studied. These ligands form interesting complexes with polymeric and oligomeric dihydrogen phosphate aggregates in the solid state. The ligands L² and L³ coordinate with H₂PO₄^– anions to form a unique molecular “necklace” with an infinite (H₂PO₄^–)_n chain and surrounding ligand molecules. Meanwhile, two different dihydrogen phosphate‐water oligomers, (H₂PO₄)₆ · (H₂O)₄ and (H₂PO₄)₄ · (H₂O)₂, were observed in the complexes with the ligands L³ and L⁴ . In addition, solution anion binding properties of the ligands were studied by ¹H NMR and UV/Vis spectroscopy.     

Ex‐Situ Kinetic Investigations of the Formation of the Poly‐Oxo Cluster U38

The ex‐situ qualitative study of the kinetic formation of the poly‐oxo cluster U₃₈, has been investigated after the solvothermal reaction. The resulting products have been characterized by means of powder XRD and scanning electron microscopy (SEM) for the solid phase and UV/Vis, X‐ray absorption near edge structure (XANES), extended X‐ray absorption fine structure (EXAFS), and NMR spectroscopies for the supernatant liquid phase. The analysis of the different synthesis batches, stopped at different reaction times, revealed the formation of spherical crystallites of UO₂ from t=3 h, after the formation of unknown solid phases at an early stage. The crystallization of U₃₈ occurred from t=4 h at the expense of UO₂, and is completed after t=8 h. Starting from pure uranium(IV) species in solution (t=0–1 h), oxidation reactions are observed with a U^IV/U^VI ratio of 70:30 for t=1–3 h. Then, the ratio is inversed with a U^IV/U^VI ratio of 25/75, when the precipitation of UO₂ occurs. Thorough SEM observations of the U₃₈ crystallites showed that the UO₂ aggregates are embedded within. This may indicate that UO₂ acts as reservoir of uranium(IV), for the formation of U₃₈, stabilized by benzoate and THF ligands. During the early stages of the U₃₈ crystallization, a transient crystallized phase appeared at t=4 h. Its crystal structure revealed a new dodecanuclear moiety (U₁₂), based on the inner hexanuclear core of {U₆O₈} type, decorated by three additional pairs of dinuclear U₂ units. The U₁₂ motif is stabilized by benzoate, oxalates, and glycolate ligands.     

Chain Folding Controlled by an Isomeric Repeat Unit: Helix Formation versus Random Aggregation in Acetylene‐Bridged Carbazole–Bipyridine Co‐Oligomers

An unprecedented, positional effect of the isomeric repeat unit on chain folding in donor–acceptor‐linked oligomers, which contain alternating bipyridine and carbazole moieties that are connected through an acetylinic linkage, is reported. 4,4′‐Linked oligomer 1 adopts an intrachain helical conformation (CD‐active) in CHCl₃/MeCN (20:80 v/v), whereas oligomer 2 , which contains an isomeric 6,6′‐linkage, forms interchain randomly coiled aggregates (CD‐inactive). The substitution position plays a significant role in controlling the variations in electronic effects and dipole moments around the bipyridyl moiety, which are responsible for this observed phenomenon. Two model compounds of oligomers 1 and 2 ( 3 and 4 , respectively) were prepared and their properties were compared. A systematic investigation of the photophysical and CD properties of these structures, as well as theoretical studies, support our conclusions.     

Preparation of Excitation‐Independent Photoluminescent Graphene Quantum Dots with Visible‐Light Excitation/Emission for Cell Imaging

We report the first pyrrole‐ring surface‐functionalized graphene quantum dots (p‐GQDs) prepared by a two‐step hydrothermal approach under microwave irradiation in an ammonia medium. The most distinct feature of the functionalized GQDs is that both the excitation and emission wavelengths fall into the visible‐light region. The p‐GQDs are excited by visible light at λ_ex 490 nm (2.53 eV) to emit excitation‐independent photoluminescence at a maximum wavelength of λ_em 550 nm. This is thus far the longest emission wavelength reported for GQDs. Stable photoluminescence is achieved at pH 4–10 with an ionic strength of 1.2 mol L^?1 KCl. These features make the p‐GQDs excellent probes for bio‐imaging and bio‐labeling, which is demonstrated by imaging live HeLa cells.     

Exciton <Emphasis Type="Italic">Cotton</Emphasis> Effects of Benzoates in the <Superscript>1</Superscript>B Transition Region. Demonstration and Applications

Summary. The benzoate ¹B region exciton Cotton effects, hitherto unexplored, were analyzed for their use in stereochemical assignments in both acyclic (conformationally flexible) and cyclic molecules. It was found that a strong, allowed ¹B_a transition, polarized longitudinally, dominates the ¹B region (185–210nm) both in the UV and the CD spectra. The exciton Cotton effects due to this transition have the same sign (but differing magnitude) as those due to the ¹L_a (CT) band. The other component of the nearly degenerate ¹B system, i.e. ¹B_b transition, polarized orthogonally to the ¹B_a transition, gives a Cotton effect in the case of di- and poly(4-chlorobenzoate) chromophoric system, the sign of which is sensitive to the configuration of di- or polyol. In rigid 5-steroid skeleton ¹B_b transition couplings appear responsible for strong exciton Cotton effects due to nearly parallelly oriented benzoate chromophores. Whereas ¹B_a transition excitation energy appears insensitive to the nature of a substituent in 4-position of the benzoate chromophore, substitution with a donor group (methoxy, dimethylamino) brings about a red shift of the ¹B_b band, although less pronounced than the red shift of the ¹L_a (CT) band.